The invention relates to apparatus and methods for tracking underwater acoustic signals and, more particularly, to an improved acoustic pulse detection and tracking system in an underwater ranging system comprising a number of hydroacoustic transmitters and receivers deployed along seismic sources and hydrophone cables for the purpose of determining their relative locations.
Hydroacoustic ranging systems are used to determine the relative locations of positions on hydrophone-arrayed seismic streamer cables and seismic sources towed behind ships performing marine seismic surveys. High-resolution surveys require accurate estimates of the locations of the seismic source and hydrophone receivers, which can be determined from the relative locations. Typical ranging systems capable of providing the required accuracy comprise a number of transmitters and receivers attached at various positions along the streamers. Individual transmitters and receivers or transceivers can also be installed on the platform supporting the seismic energy source, on head-end buoys, and on tail-end buoys. Ranges between pairs of transmitter and receiver locations are measured by the transit times of pulses transmitted at more or less regular intervals by the transmitters and received by the receivers. The ranges or the transit times are reported to a host computer for on-line estimation of the array and source locations or for storage for later off-line processing.
Commercial ranging systems are manufactured by Sonardyne of Hants, United Kingdom, Syntron, Inc. of Houston, Tex., and DigiCOURSE, Inc. of Harahan, La., a wholly-owned subsidiary of the assignee of this invention. Many of the details of the DigiCOURSE system for streamer location estimation are described in U.S. Pat. No. 5,031,159, issued Jul. 9, 1991, in the name of Robert E. Rouquette. Although the ranging system of Rouquette operates well, certain adverse conditions can cause problems. For example, in high sea states, the acoustic noise level increases relative to the signal level, making detection more difficult, especially at longer ranges. Variations in sea state and ambient noise conditions also degrade system performance.
Another significant problem is caused by the bubble curtain generated by the seismic source, the ship's wake, or propeller cavitation. For, example, the collapse of the large air bubble produced by the seismic blast sheds millions of small bubbles that trail the seismic source platform in the form of a bubble curtain. Consistent ranges through the bubble curtain are difficult to obtain because the bubbles attenuate the acoustic pulses and strengthen reverberation and forward scattering, thereby degrading pulse detection. The Sonardyne and the Syntron systems attempt to solve the problem by transmitting sequences of acoustic pulses and determining reception quality by the number of pulses from the sequence successfully received. One problem with that solution is that the total time required to transmit and receive the complete sequence of pulses is long and usurps valuable time needed with large acoustic networks to obtain all the necessary ranges. Furthermore, transmitter and receiver duty cycles are long, leading to increased power consumption and decreased battery life in typical battery-powered applications.
To solve the foregoing problems, there is a need for an improved acoustic ranging system usable in large acoustic networks and capable of producing consistent, high-quality ranges in a variety of adverse operating conditions, especially those characterized by high or varying ambient noise or by severe signal attenuation, reverberation, or forward scattering.